This invention relates to the field of CRT devices and, more particularly, to the correction of pincushion distortion in such a device.
In an electromagnetically deflected cathode ray tube, the sine of the deflection angle is proportional to the deflection current. Therefore, an undistorted rectangular pattern appears on the screen of a tube only when the radius of curvature is equal to the length of the beam, measuring from the center of the deflection plates. Since most-present day CRT face plates are flat, or nearly so, a so-called "pincushion" distortion occurs, meaning that the corners of the raster are stretched out. Due to the aspect ratio of the raster, there can be theoretically 1.33 times as much distortion along the top and bottom edges as there is along the sides. Distortion may also be different at the top and bottom of one CRT due to minor misalignment of the yoke for convergence purposes. In some CRT devices, pincushion distortion is partially prevented by special design of the yoke winding and, in black and white tubes, the yoke design may correct it sufficiently. In color CRT's, however, this method may cause degradation of the spot size and convergence and thus reduce resolution at the corners of the screen. This effect is particularly undesirable in the use of the CRT's used in data terminals, as resolution must be of high quality over the entire screen. Another correction method involves the use of an extraneous electrostatic or electromagnetic field in the area between the yoke and the screen. Spot distortion with this method can be minimized by placing the field close to the screen. A better approach is to decrease the rate of change of sweep circuit current as it approaches its peak values. This latter method can produce satisfactory results, but the current function is not a simple one to provide and the typical implementation requires a reactor transformer and power drive circuitry at considerable added expense.